1. Technical Field of the Invention
This invention relates generally to the field of treating subterranean formations to increase the production of oil and/or gas therefrom. In particular, the invention relates to a novel process to stimulate the hydrocarbon production zone of a single or multi lateral long horizontal hydrocarbon well by acid treatment.
2. Description of the Prior Art
Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the oil to reach the surface. In order for oil to be “produced,” that is travel from the formation to the wellbore (and ultimately to the surface) there must be a sufficiently unimpeded flowpath from the formation to the wellbore. This flowpath is through the formation rock—e.g., sandstone, carbonates—which has pores of sufficient size and number to allow a conduit for the oil to move through the formation.
One of the most common reasons for a decline in oil production is “damage” to the formation that plugs the rock pores and therefore impedes the flow of oil. This damage generally arises from another fluid deliberately injected into the wellbore, for instance, drilling fluid. Even after drilling, some drilling fluid remains in the region of the formation near the wellbore, which may dehydrate and form a coating on the wellbore. The natural effect of this coating is to decrease permeability to oil moving from the formation in the direction of the wellbore.
Another reason for lower-than-expected production is that the formation is naturally “tight,” (low permeability formations) that is, the pores are sufficiently small that the oil migrates toward the wellbore only very slowly. The common denominator in both cases (damage and naturally tight reservoirs) is low permeability. Techniques performed by hydrocarbon producers to increase the net permeability of the reservoir are referred to as “stimulation.” Essentially, one can perform a stimulation technique by: (1) injecting chemicals into the wellbore to react with and dissolve the damage (e.g., wellbore coating); (2) injecting chemicals through the wellbore and into the formation to react with and dissolve small portions of the formation to create alternative flowpaths for the hydrocarbon (thus rather than removing the damage, redirecting the migrating oil around the damage); or (3) injecting chemicals through the wellbore and into the formation at pressures sufficient to actually fracture the formation (hydraulic fracturing), thereby creating a large flow channel though which hydrocarbon can more readily move from the formation and into the wellbore.
When a hydrocarbon-bearing, subterranean reservoir formation does not have enough permeability or flow capacity for the hydrocarbons to flow to the surface in economic quantities or at optimum rates, hydraulic fracturing or chemical (usually acid) stimulation is often used to increase the flow capacity as described above. A wellbore penetrating a subterranean formation typically consists of a metal pipe (casing) cemented into the original drill hole. Lateral holes (perforations) are shot through the casing and the cement sheath surrounding the casing to allow hydrocarbon flow into the wellbore and, if necessary, to allow treatment fluids to flow from the wellbore into the formation.
There are generally two types of acid treatment: fracture acidizing (injection of acid at rates above fracture pressure to etch the faces of the resultant fractures) and matrix acidizing (injection of acid at rates below fracture pressure to dissolve flow channels in the rock or to remove scale or damage caused by drilling). Acid treatments are employed in all types of oil wells and occasionally in water wells: they may be used to open fractures or remove damage in newly drilled wells or to rehabilitate old wells from which production has declined.
Hydraulic fracturing consists of injecting viscous fluids (usually shear thinning, non-Newtonian gels or emulsions) into a formation at such high pressures and rates that the reservoir rock fails and forms a plane, typically vertical, fracture (or fracture network) much like the fracture that extends through a material as a result of a wedge being driven into it. Granular proppant material, such as sand, ceramic beads, or other materials, is generally injected with the later portion of the fracturing fluid to hold the fracture(s) open after the pressures are released. Increased flow capacity from the reservoir results from the more permeable flow path left between grains of the proppant material within the fracture(s). In chemical stimulation treatments, flow capacity is improved by dissolving materials in the formation or otherwise changing formation properties.
Aqueous acid solutions, acid-like fluids or fluid of similar function are commonly used to treat oil or gas wells. For example, subterranean well formations are often contacted with aqueous acid solutions to increase the permeabilities of the formations whereby the production of oil and/or gas therefrom is increased. Aqueous acid solutions are also utilized to etch flow channels in the faces of fractures formed in the formations and to clean out perforations and tubular goods in wells.
Formation damage removal and effective stimulation of horizontal wells are known industry challenges. It is also known to stimulate vertical wells using acid treatment, also called acid stimulation. Horizontal wells, in addition to the greater length, experience increased possible difficulties during acid treatment involving coil tubing units or CTU. Selection of the appropriate acid is made applying criteria known in the art.
Acid treatment is used during the cleaning of new long horizontal hydrocarbon wells or to increase the productivity of such long horizontal wells. The conventional method of acid treatment is to use a Coil Tubing Unit (CTU) such that the acid treatment fluids is pumped through the CTU which is extended into the wellborn such that the acid treatment fluids are introduced into the hydrocarbon production zone in order to clean it out and increase permeability. This technique can be used for oil, gas, and water wells. The limitations in this method relate to the length of the well and mechanical limitations introduced through the insertion of the CTU. For example, acid treatment is generally considered ineffective for long wells, such as those in excess of one kilometer.
Acids useful in such stimulation or acid treatment processes are typically extremely active, such as hydrofluoric acid. Therefore, methods of acid treatment have typically diverged greatly from other methods of injection due to the nature of the acid.
Means of injecting substances other than acid have been described. For example, U.S. Pat. No. 4,262,745 teaches the injection of steam when two vertical wells are connected subsurface by a conduit or other means near the surface. This is intended to stimulate heavy oil producing wells using steam. This subsurface conduit serves the single purpose of allowing the steam to be injected and is not otherwise a useful wellbore. It also is not intended to carry erosive or fracture fluids such as those used in acid stimulation.
It would be advantageous to provide a means for cleaning of new wells such that the entire length of the wellbore is cleaned. It would be advantageous to also provide a means for such cleaning that is effective for long horizontal wells. It would be particularly advantageous to provide a method wherein acid treatment can be used.
It would be advantageous to provide a means for stimulating production of the formation. It would be advantageous to also provide a means for such stimulation that is effective for long horizontal wells.